Investigations of the proximity effect for pollutants in the indoor environment

More than a dozen indoor air quality studies have reported a large discrepa ncy between concentrations measured by stationary indoor monitors (SIMs) an d personal exposure monitors (PEMs). One possible cause of this discrepancy is a source proximity effect, in which pollutant sources close to the resp ondent cause elevated and highly variable exposures. This paper describes t hree sets of experiments in a home using real-time measurements to characte rize and quantify the proximity effect relative to a fixed distant location analogous to a SIM. In the first set of experiments, using sulfur hexafluo ride (SF6) as a continuously emitting tracer pollutant From a point source, measurements of pollutant concentrations were made at different distances from the source under different air exchange rates and source strengths. A second set of experiments used a continuous point source of carbon monoxide (CO) tracer pollutant and an army of high time resolution monitors to coll ect simultaneous concentration readings at different locations in the room. A third set of experiments measured particle count density and particle-bo und polycyclic aromatic hydrocarbon (PAH) concentrations emitted from a con tinuous particle point source tan incense stick) using two particle counter s and two PAH monitors, and included human activity periods both before and during the source emission period. Results from the SF6 and CO experiments show that while the source is emitting, a source proximity effect can be s een in the increases in the mean and median and in the variability of conce ntrations closest to the source, even at a distance of 2.0 m from the sourc e under certain settings of air exchange rate and source strength. CO conce ntrations at locations near the source were found to be higher and more var iable than the predictions of the mass balance model. For particles emitted from the incense source, a source proximity effect was evident for the fin e particle sizes (0.3 to 2.5 mu m) and particle-bound PAH up to at least 1. 0 m from the source. Analysis of spatial and temporal patterns in the data for the three tracer pollutants reveal marked transient elevations of conce ntrations as seen by the monitor, referred to as "microplumes," particularl y at locations close to the source. Mixing patterns in the room show comple x patterns and directional effects, as evidenced by the variable intensity of the microplume activity at different locations. By characterizing the sp atial and temporal variability of pollutant concentrations in the home, the proximity effect can be quantified, leading to improved indoor monitoring designs and models of human exposure to air pollutants.